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Discussion Forum

1/2″ PEX same as 1/2″ copper, etc?

WillieWonka | Posted in General Discussion on August 22, 2008 07:49am

I’ve been moving more and more to using PEX for my customers and yet I have a question that keeps popping in my mind. Is 1/2″ PEX the same as 1/2″ copper in terms of flow rates, pressure, etc? Same with 3/4″ PEX, is it the same as 3/4″ Copper?

I ask because the fittings for PEX that you crimp to are noticably smaller in diameter. I didn’t meaure it but it almost looks like a 1/2″ PEX fitting has an opening that is like 3/8″ ID which to me is not the same as 1/2″ copper.

A side question to this then is, when planning an installation with PEX, because of the above, do you have to calculate pipe size vs. fixture units differently?

I been in a few houses recently that plumbed with 1/2″ just about everywhere and wondered if it’s still somehow equivalent to copper.

If at first you don’t succeed, try using a hammer next time…everything needs some extra persuasion from time to time.  -ME
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Replies

  1. User avater
    BillHartmann | Aug 22, 2008 10:15pm | #1

    Read this thread and then ask your questions.

    http://forums.taunton.com/tp-breaktime/messages?msg=108596.1

    While it has fore 3/8" some of the information is the same and it has links to design manuals and pressure drop calculations.

    In general the fittings only add an equivalent of couple of feet of PEX. And in general there are far fewer fittings and smoother tubing than copper so the flow is the same or maybe even better.

    .
    .
    A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
    1. WillieWonka | Aug 23, 2008 02:57am | #3

      That was an interesting thread. I never considered how pressure would drop with distance due to fittings and the like. I was a bit perplexed, though. What was the final answer on homerun vs. non-homerun (using a manifold)? I was always told homerun it but could never understand why no one ever talked about the manifolds I kept seeing. I always thought pressure would go UP with a smaller pipe. Learn something new every day. That's what I like about this business, I an constantly refining my skills and knowledge to become more valuable in my field. Anyways, in my case, if I understood the thread correctly, a 1/2" PEX ftg will NOT reduce pressure by much and as you say since there are longer runs and less ftgs the pressure drop and flow rate ultimately worksout to be about the same, correct?So in essense, 1/2" PEX is pretty much the same as 1/2" copper in spite of the smaller orifices of the ftgs.If at first you don't succeed, try using a hammer next time...everything needs some extra persuasion from time to time.  -ME

      1. User avater
        BillHartmann | Aug 23, 2008 03:07am | #5

        I don't think that there was every AN answer on home run vs manifold.To many variables to come out with a simple statement..
        .
        A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

  2. User avater
    xxPaulCPxx | Aug 23, 2008 02:03am | #2

    That 3/8"constriction canbite you inthe butt if you use 1/2" pex between the shower valve and the tub spigot.  The reduced flow will cause water to flow out the shower head as well as the tub spout.

    Rebuilding my home in Cypress, CA

    Also a CRX fanatic!

    Save the Whales! and Guns!

    1. brucet9 | Aug 23, 2008 02:59am | #4

      "That 3/8"constriction can bite you in the butt if you use 1/2" pex between the shower valve and the tub spigot. The reduced flow will cause water to flow out the shower head as well as the tub spout."Can you explain that? How will 1/2" Pex back water up the riser when 1/2" copper will not?The pressure head of water in a typical shower head riser will be about 1.75psi. It doesn't seem like a couple of Pex fittings between valve and spout would cause that much resistance to flow.
      BruceT

      1. User avater
        xxPaulCPxx | Aug 23, 2008 07:45pm | #6

        Oh, but it does.  The 1/2 pex tube has an inner support for even compression of the tube at each fitting connection.  The inside diameter of these connections is 3/8".

        Remember, the water stop in the spout that triggers the flow into the shower head isn't a real valve.  All it does is impead the flow.  Back in the mixing valve, it senses the restriction in flow to trip over to the shower head.  Having two 3/8" constrictions in the tube between the valve and the spout is enough to partially trigger the valve.  The result is that when you fill the tub, water will flow out the shower head too.

        I fixed it by pulling out my pex connection between the mixing valve and spout.  You can still use pex on that connection, but it has to be 3/4" pex instead of 1/2".

        Rebuilding my home in Cypress, CA

        Also a CRX fanatic!

        Save the Whales! and Guns!

        Edited 8/23/2008 12:48 pm by xxPaulCPxx

        1. MSA1 | Aug 23, 2008 07:51pm | #7

          Thats what hapened at one of my rentals. I'll have to let my plumber know that.

          Whats the fix? Simply running 3/4" pex between the two? 

          Family.....They're always there when they need you.

          1. User avater
            xxPaulCPxx | Aug 24, 2008 05:10am | #11

            Yes, either in copper or just with threaded brass tube to the eared stubbout.Rebuilding my home in Cypress, CA

            Also a CRX fanatic!

            Save the Whales! and Guns!

          2. MSA1 | Aug 24, 2008 05:14am | #12

            Too late on this one, but next time......:>) 

            Family.....They're always there when they need you.

        2. User avater
          BillHartmann | Aug 23, 2008 09:14pm | #8

          Since you need hard mounts for the mixer valve adn the spount anyway it seem that it more work to run them in plex than in copper..
          .
          A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

        3. brucet9 | Aug 24, 2008 01:11am | #9

          "Back in the mixing valve, it senses the restriction in flow to trip over to the shower head."What? I don't think there is any such pressure sensor-controlled diverter in tub/shower valves. That seems way too complicated when all they have to do is provide two outlets and let pressure head decide which one flows. Most valves look something like this one : http://www.plumbersurplus.com/Prod/Mixet-964-R-O-4-port-1-2-F-I-P-Tub-Shower-Rough-Valve-Only/31637/Cat/281
          or this: http://www.plumbersurplus.com/Prod/Moen-Showhouse-S932-M-PACT-Valve-Rough-in-Posi-Temp-pressure-balancing-cycling-valve-with-stops/63882/Cat/281 and I can't see any room for a sensor-controlled diverter, much less any way to install it into that small brass casting.BruceT

          1. User avater
            xxPaulCPxx | Aug 24, 2008 05:08am | #10

            Pretty amazing wht they can machine into such a small space.  In your second example, where do they put the temp-pressure-balancing-hardware?  I don't have an answer for you, as I've never pulled on apart.

            Do a search here on this topic Pex and Tub and Shower and you should find a previous thread on this from earlier this year.

            Another thing you can do is look at tub faucet that has the cutoff for the shower built into it.  You should see that it's about as water tight as a hallway door.

            Hey, if anyone else has a different explination they will post it.  People here have no problems telling eachother they are wrong :)Rebuilding my home in Cypress, CA

            Also a CRX fanatic!

            Save the Whales! and Guns!

          2. User avater
            Dreamcatcher | Aug 24, 2008 05:41pm | #13

            "People here have no problems telling eachother they are wrong :)"People here have problems being told they are wrong ;)Anyway, I just wanted to jump in and ask some of you PEX pros what the avg. cost is for PEX vs. Copper? Taking into account the PEX manifold, individual lines, labor hours, and the average 2000sf house with two bathrooms and a kitchen. Whenever I see the scrapper trucks out or hear about the price of copper these days, I wonder if it will ever become viable to be worthwhile to replace houses full of copper with PEX? As I am in the process of remodeling my own home today, I kick myself for not having converted to PEX in the beginning. I replumbed a considerable portion in copper, then tried PEX to add a couple of outside spigots. I don't even want to use copper anymore considering the price and labor and wonder if it would be worth stepping back a little to install a manifold and individual lines. I have just one more bathroom to plumb in and debating whether to go copper or suddenly convert to PEX. Another quick question...does running an individual PEX line to a shower get the HOT water there any quicker? right now takes about 45sec. to get warm enough to enter. The run is about 34ft. 3/4" copper line (55psi) uninsulated in a conditioned crawlspace.gk

          3. marv | Sep 04, 2008 03:55pm | #16

            does running an individual PEX line to a shower get the HOT water there any quicker?

            No since the water comes the same distance from the water heater.  If you run 3/4 to a manifold near the bathroom, when you run hot water to sink or tub, they both will have hot water.  If you use home runs, you will have to run both for a time to bring hot water up.You get out of life what you put into it......minus taxes.

            Marv

          4. User avater
            xxPaulCPxx | Sep 04, 2008 08:31am | #14

            I just wanted to give an update here, in relation to information I posted before...I WAS WRONG!!!I happened to have my tub spout off recently, and I decided to verify my claim that "A tub spout is a tight a seal as a hallway door". I put the spout up to my mouth and blew as I closed the valve to trigger the shower. To my surprise, the seal was nearly airtight.I wanted to tell you specificly was that I was wrong in what I said, and you were correct. The entire tub to shower trigger happens in the tub spout, not the mixing valve as I had thought.You were right, I was wrong.I bet you never expected to hear that on the internet!Rebuilding my home in Cypress, CAAlso a CRX fanatic!

            I don't feel it's healthy to keep your faults bottled up inside me.

          5. MikeHennessy | Sep 04, 2008 02:39pm | #15

            "I don't feel it's healthy to keep your faults bottled up inside me"

            Had to read it twice, but great line. I gotta remember that one.

            Mike HennessyPittsburgh, PA

          6. User avater
            xxPaulCPxx | Sep 04, 2008 07:00pm | #19

            It's from the "Get Fuzzy" comic strip. The cat has some of the best lines ever.Rebuilding my home in Cypress, CAAlso a CRX fanatic!

            I don't feel it's healthy to keep your faults bottled up inside me.

          7. brucet9 | Sep 04, 2008 06:25pm | #18

            "I put the spout up to my mouth and blew as I closed the valve to trigger the shower. To my surprise, the seal was nearly airtight."Thanks for the update. I didn't think the seal would have been THAT good.By the way, did you have a good story ready in case anyone walked in on you and found you blowing into a tub spout?
            BruceT

          8. User avater
            xxPaulCPxx | Sep 04, 2008 07:03pm | #20

            Wife walks into the bathroom and sees me doing my best Dizzy Gilespy on the tub spout.She stares at me.I say "What, you can't hear that?"To which she replies "But why aren't you wearing pants?"Rebuilding my home in Cypress, CAAlso a CRX fanatic!

            I don't feel it's healthy to keep your faults bottled up inside me.

          9. brucet9 | Sep 05, 2008 03:32am | #21

            LOLNo way to top that little story.
            BruceT

  3. marv | Sep 04, 2008 03:57pm | #17

    I ask because the fittings for PEX that you crimp to are noticably smaller in diameter.

    For this reason, I believe Wirsbo (Uponor) is a surperior product.  It uses expander fittings.  The drawback is the higher cost of tools.

    You get out of life what you put into it......minus taxes.

    Marv

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